The genetic landscape faced by a living cell is constantly changing. Developmental transitions, environmental shifts, and pathogenic invasions lead to a dynamic character to the genome. Experiments in which foreign RNA or DNA are deliberately introduced into cells provide us a window with which to study general responses to genetic change. Such approaches have allowed the characterization of a number of chemical features that alert the cell to unwanted genetic activity.

One of these features is double stranded RNA. Absent during 'normal' gene expression, double stranded RNA is an essential component in the life cycle of most viruses and many other unwanted invaders. By scrupulously avoiding the production of double stranded RNA during most normal gene expression, the cell can use this chemical signature as a signal to invoke protective mechanisms. The mechanism by which cells protect themselves from unwanted RNA is of interest as a fundamental biological process, as a basis for effective genetic tools, and as an example of a structure-based defense mechanism at a subcellular level.